1. FIELD OF INVENTION
The present invention relates to a recording sheet having a surface layer of microcapsules and more particularly to an imaging sheet employing a layer of photosensitive microcapsules wherein improved cohesion of the microcapsules to one another and to the substrate is provided.
2. DESCRIPTION OF PRIOR ART
Imaging systems employing photosensitive microcapsules are described in U.S. Pat. Nos. 4,399,209 and 4,440,846 to The Mead Corporation. While the adhesion problems addressed herein have arisen in transfer imaging systems of the type described in U.S. Pat. NO. 4,399,209, the modifications which are proposed are also applicable to the self-contained imaging systems of U.S. Pat. No. 4,440,846. Transfer systems, utilize a photosensitive sheet known as a donor sheet which is prepared by coating a layer of photohardenable microcapsules on a support. The donor sheet is image-wise exposed to actinic radiation, assembled with a developer sheet and passed through the nip between a pair of pressure rollers. Images are formed by exposure-controlled release of an image-forming agent contained with a photohardenable composition in the microcapsules. The image-forming agent is typically a substantially colorless color precursor which reacts with a developer to form a visible image.
A need has arisen for improved cohesion of the microcapsules to one another. A number of factors have contributed to this need. First, some of the binders which have previously been used, like polyvinyl alcohol (PVA), do not provide good adhesion under higher humidity conditions, e.g. 25.degree. C., 85% RH. It has also been found that higher pressures and slower roller speeds may be desirable in rupturing microcapsules--this combination of forces increases the adhesion of the microcapsules to the pressure roll and causes delamination of the microcapsules when the donor sheet and the developer sheet are separated.
The microcapsules are also preferably coated on highly reflective metallic substrates which may have low adhesion for the microcapsules. A particularly preferred reflective substrate is a material such as an aluminized film. These films are extremely hydrophilic and do not bind a layer of microcapsules well especially under humid conditions.
Finally, in some processes, the donor sheet may be reversed and rewound between exposures to eliminate waste between successive images. As a result, portions of the donor sheet may be traversed by the pressure rollers as many as three times, thus further accentuating the need for better adhesion and cohesion of the layer of microcapsules.
The delamination problem is exacerbated in that the developer materials which are typically used are low molecular weight phenolic resins which are somewhat tacky. When the microcapsule sheet is exposed and assembled under pressure with a developer sheet coated with a phenolic resin to form the image, the microcapsules can be picked off the substrate. Attempts have been made to address microcapsule delamination in the prior art. Japanese KOKAI No. 62-176893 to Canon teaches a transfer recording medium comprised of a substrate and microcapsules disposed thereon, with the microcapsules or their coatings fixed to a surface material by means of a chemical reaction. Microcapsules are disposed on a polyethylene terephthalate film which is integrated with the wall material of the microcapsules by thermal polymerization, photopolymerization, condensation polymerization, crosslinking or the like. A substance which effects thermal polymerization or photopolymerization is included in the microcapsule walls and substrate or a surface coating and may include a thermal polymerization or photopolymerization initiator, a polymeric prepolymer, a polymeric oligomer and a crosslinking agent such as ethylene glycol diacrylate, propylene glycol diacrylate or 4-butanediol diacrylate.